Recently, for reducing traffic accidents, the ITS (Intelligent Transport System) has been under development. As such an ITS system, there is an embodiment of the road-to-vehicle communications in which the traffic signal is provided with a wireless base station (road side device), and also in which information, such as traffic signal information (the signal light is red or green, or the like) and the presence or the absence of a right-turning vehicle, is set to a terminal (on-vehicle device) provided for an approaching vehicle that is present several meters ahead.
On the other hand, recently, as an intermediate distance large-capacity wireless communications system, standardization and development of a system called WiMAX have been progressed in the IEEE802.16WG (Working Group) by use of the OFDMA (Orthogonal Frequency Domain Multiple Access) scheme in which multiplexing in the frequency axis direction and the time axis direction to the wireless frame is flexibly performed. In this instance, the IEEE802.16WG mainly defines two types of standards, that is, the IEEE802.16d for use in fixed communications (for example, see the following non-patent documents 1 and 2) and the IEEE802.16e for use in mobile communications (for example, see the following non-patent document 2).
The IEEE802.16d and the IEEE802.16e define that a wireless terminal (MS: Mobile Station) realizes communications between the BS and the MS in accordance with the MAP information contained in the wireless frame sent by the wireless base station (BS: Base Station).
FIG. 8 illustrates a wireless frame format in conformity to the IEEE802.16e standard. As depicted in FIG. 8, a wireless frame in conformity to the IEEE802.16e standard is subjected to time-division multiplexing of a downlink (DL) sub-frame in the direction from the MS to the BS and an uplink (UL) sub-frame in the direction from the MS to the BS. In this instance, as to the time axis direction, a TTG (Transmit Transition Gap) is provided between the DL sub-frame and the UL sub-frame; an RTG (Receive Transition Gap) is provided between the wireless frames (between the UL sub-frame and the subsequent wireless frame).
Further, to the DL sub-frame, a preamble, a frame control header (FCH), downlink MAP information (DL-MAP), uplink MAP information (UL-MAP) (hereinafter, these will be also called header information), and one or more downlink bursts (DL bursts), are multiplexed in the two-dimensional domain formed by the time axis (symbol time) direction and the frequency (frequency channel) direction. To the UL sub-frame, one or more uplink bursts (UL bursts) are multiplexed. In this instance, the number of DL bursts is defined to be 15 at maximum. Yet further, information other than the UL bursts is generated by the BS.
Here, the preamble is a field into which frame synchronization information is inserted, and the FCH is a field into which information relating to the MAP such as the size and the position of the MAP is inserted. Further, the MAP information includes information such as the communications connection ID (CID) sent with wireless resources [frequency channel and time (transmission timing)] (this is called “burst”) used by the MS in communications, the burst allocation position (burst position) of the burst of the connection in the wireless frame, the size of the burst (burst size), the modulation scheme of the burst (QPSK, 16QAM, 64QAM, or the like), coding ratio, or the like.
That is, the MAP information is regarded as information (burst allocation information) which specifies (allocates) the field (reception field and transmission field) of the wireless frame to be received and sent by the MS. In this instance, the abovementioned burst position can be specified with symbol offset and sub-channel offset from the leading symbol of the wireless frame; the above mentioned burst size can be specified by the number of symbols and the number of sub-channels.
Accordingly, the MS detects the above preamble, thereby establishing downlink and uplink wireless frame synchronization. Further, the MS performs demodulation processing and decoding processing to the DL burst field specified by the DL-MAP defined by the FCH, thereby executing selective reception processing to the DL bust distained to the local MS, while performing data transmission to BS in the UL burst field specified by UL-MAP.
Here, since there is a necessity in the ITS for sending a large amount of data, such as an image data, from a road side device to an on-vehicle device, application of the ITS, as a wireless communications system in which large-amount and flexible multiplexing is available, to road-to-vehicle communications in the WiMAX system based on OFDMA is now under view.
Non-Patent Document 1: IEEE802.16-2004
Non-Patent Document 2: IEEE802.16e-2005 (IEEE 802.16-2004/Cor1-2005)